The present invention generally relates to a radio frequency selection device in a radio communication system in which a radio communication channel is established between a mobile station and a base station by allocating a radio communication frequency band different from other radio communication system frequency bands, specifically relates to such a radio frequency selection device, a radio communication system and a radio control channel establishing method that can realize high quality transmission of the control information, highly accurate control and effective use of frequency resources.
Recently, demands for mobile communication services are rapidly increasing, and it is desired to realize high speed and large capacity communication services such as motion video or high speed data transfer, as well as conventional audio or low speed data transfer. Accordingly, in future mobile communication services, broad frequency bands should be secured to support low speed data communication to high speed and large capacity data communication.
However, frequency resources are limited and it is difficult to secure broad frequency bands, and therefore it is desired to provide communication methods capable of utilizing frequencies effectively. Radio communication systems have been already developed in a variety of ways, and users can receive a variety of services. However there is a problem in that service areas are not secured well enough when a new system is introduced, as when a new radio communication system is proposed and commercially utilized, in which plural different frequency bands are used for the effective use of frequency and broad service areas (See Non-Patent Document No. 1).
In the PDC (Personal Digital Cellular) system, for example, both bands of 800 MHz and 1.5 GHz are used (See Non-Patent Document No. 2). Mobile stations and base stations in this PDC system can utilize radio channels belonging to 800 MHz band or 1.5 GHz band. In this method, during a waiting mode or an audio communication operation mode, a frequency channel is allocated from candidates belonging to any band of 800 MHz or 1.5 GHz. In this manner, it is possible to shift to the 1.5 GHz band from the 800 MHz band in which traffic is extremely congested. A frequency band used for waiting by a mobile station is selected among these two bands, based on band transfer probability reported by the network. With respect to audio communication operations, the mobile station measures radio wave conditions of both frequency bands to secure available frequencies, and selects an adequate frequency band that could satisfy a desired quality and maintain the communications.
Non-Patent Document No. 3 proposes a network structure and a control method that integrates each system seamlessly in case where plural different radio communication systems are established in the same area. In this network structure, different radio communication systems and base stations belonging thereto are constructed independently in accordance with each environment. Mobile stations can be connected to any of the systems, and can establish basic access network for transmitting control signal among the systems.
By controlling plural communication channels, it becomes possible to utilize frequency resources effectively, secure service areas and improve throughput.    [Non-Patent Document No. 1]            R. Heichkero, et al., “Ericsson Seamless Network, “Ericsson Review, No. 2, pp. 76-83, 2002.            [Non-Patent Document No. 2]            “1.5 GHz band common usage. System Summary” by Ikeshita, Fukazawa and Masuda, NTT DoCoMo Technical Journal, Vol. 10, No. 1, pp. 6-14, April, 2003.        “MIRAI Architecture for Heterogeneous Network,” by G. Wu, et al., IEEE Communication Magazine, No. 2, pp. 126-134, February 2002.        
However, the above explained related art examples have the following problems.
Although the above systems can integrally manage radio channels for transmitting control signal, it is necessary to construct a new base access networking addition to the already existing radio channels. It is also necessary to prepare enough line capacity for transferring control information.
It is difficult to secure an adequate radio control channel for each system in consideration of required control information amount. Generally, as the frequency band becomes higher, available signal bandwidth becomes broader and a user information transmission amount becomes larger. In this situation, an amount of control information necessary for maintaining communication is expected to become larger as the user information transmission amount. However, in the basic access network, it is difficult to flexibly secure radio control channels in accordance with the varied control information transmission amount.
In the above prior examples, the control is performed based on measured results of received signals, but frequency characteristics of radio transmission signals are not considered.
When establishing radio control channels using different frequency bands, differences in information amounts between the user information and the control information, and coverage area to which the control information is transmitted are not considered. In practice, it is necessary to consider propagation characteristics in the real environment, fading effect, difference in securable frequency bandwidth, information amount difference between the user information and the control information, quality required for the control information and coverage area to which the control information is transmitted.